Recently compact audio disk systems have been introduced into the market place. These systems use a small disk, approximately 12 centimeters in diameter into which digitized audio information has been pressed in a spiral information track. The playback unit utilizes a laser beam focused on the track to read out the digitized information. The read out information is then converted to an analog audio signal and fed to conventional audio amplifiers and speakers.
It was soon realized that these compact disks could also be used for mass storage of digital information for a computer. In this application, the device is referred to as a compact disk read only memory (CD-ROM). A single compact disk can contain 19 billion bits (approximately 2.5 billion bytes) of digital information. The data are recorded on a master disk at a constant linear velocity and a constant length for each recorded bit. As a result, each convolution of the spiral track contains a different number of bits.
Unlike the typical playback of an audio record, the access of data from a CD-ROM does not necessarily occur in the same sequence as it was recorded. For random access applications, the playback head jumps to various points on the spiral track to read out the desired data. For example, if video information for a game is stored on a disk, the next scene to be displayed may vary depending on a selection by the game player (e.g., which door gets opened or which path is chosen). The selection may require a scene which is stored several convolutions of the spiral track from the present location of the head. The head will then rapidly move radially jumping across several convolutions on the disk to the desired one.
However, once the head completes the rapid radial movement and has synchronized to the recorded data, the system may have to wait up to one full revolution of the disk before the beginning of the desired data is located under the pickup head. This time delay between when the head is ready to read and when the data are actually present under the head is known as rotational latency. Although this delay is a fraction of a second, it may be a relatively long interval in terms of computer speed. In order to minimize the rotational latency, the data could be recorded in the spiral track beginning at the exact location where the head will be when it is ready to read out the next scene. Considering the very large storage capacity of the compact disk, one need not be concerned with economizing the recording space and there may be gaps between different items of data in order to precisely locate the data for optimal access. In the above game example, the scenes for each path selection would be positioned on the disk so that they would be immediately available when the playback head jumps to them from the branch position.
However, this minimization of rotational latency requries that one know exactly where on the disk to record the information and the use of a highly controlled recording process. In principle, the location of each bit in the track could be calculated and used by the computer to optimize the data access. However, since the speed of the disk and the rate at which the data are recorded are independent of each other, variation of one or the other parameter will unpredictably vary the location of a given bit from its calculated position.
Although various recording systems have been developed for maintaining the speed of the rotating disk at a constant linear velocity, such systems have not been keyed to the rate of the data being recorded. Therefore, although these systems provide an approximately constant linear velocity during recording, they are not accurate enough for reducing the rotational latency as discussed above.